(a) Technical Field
The present invention relates to a fuel cell bipolar plate, which prevents water accumulation by locally increasing heat transfer resistance and temperature with the use of a layer or subsection having a low thermal conductivity, thus preventing the occurrence of flooding.
(b) Background Art
Catalyst or diffusion media flooding (hereinafter referred to as “flooding”) of a fuel cell occurs as liquid water is accumulated at the bottom of land portions 12a of a bipolar plate 100 or along the wall surface of a gas flow field during operation of the fuel cell as shown in FIG. 1. Such flooding makes it difficult for hydrogen and air to reach the catalyst layer, thus reducing the performance of the fuel cell during operation in a normal state. Flooding can also be responsible for performance instability and hasten deterioration of component life, reducing durability.
In a structure in which the flooding is more likely to occur, the amount of water remaining in the fuel cell during shut-down of the fuel cell is relatively larger than a structure in which the flooding is less likely to occur. When the fuel cell is kept at sub zero temperatures, the amount of generated ice is increased with the increase in the amount of remaining liquid water, which deteriorates the durability of the fuel cell and requires more energy and time to restart the fuel cell.
Accordingly, it is necessary to minimize the amount of liquid water so as to improve the durability of the fuel cell.
In order to prevent the flooding in the fuel cell, the temperature of an outlet port of an air flow field of the bipolar plate, where the amount of water is relatively larger, can be designed to be maintained at a relatively high temperature.
In some cases where the coolant temperature is relatively low, flooding is most likely to occur. Because of the mismatch between the heat transfer through the channel gas, which is low, and the bipolar plate, which is high, flooding is most likely occur under the land portions. Accordingly, there are difficulties in preventing the occurrence of flooding merely using temperature distribution of the inlet and output ports due to the variation in heat transfer characteristics between the channel and land locations.
Moreover, during operation, a transient flow increase is performed periodically or when the performance of the fuel cell is deteriorated to remove the liquid water present on the flow field. Furthermore, purging is performed to reduce the amount of water during shut-down. These operations are parasitic and reduce efficiency, but are needed to reduce liquid water accumulation detrimental to performance, stability, and durability.
However, in the case where the material having the same heat transfer characteristic as the bipolar plate is used in designing the coolant flow field of the bipolar plate, it is difficult to locally control the temperature distribution of the bipolar plate, and thus there are difficulties in preventing the occurrence of flooding. Moreover, it is difficult to remove the liquid water accumulated under the land portions by the purging operation.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.